Natural swimming pools: introduction

“With a minimum of materials and without an arsenal of chemicals, you can build an idyllic water oasis right in your own back yard and thwart summertime’s sultry dog days.” – Mother Earth News

What are natural swimming pools?

They’re constructed pools / ponds designed for swimming, but with room for nature. They typically have a swimming area and a ‘regeneration zone’, where the water is cleansed biologically (by plants and micro-organisms) and with physical filters, but without chlorine, ozone, copper compounds, UV radiation or ultrasonic devices, which are undesirable because they reduce the biological activity that a natural pool is intended to encourage. Microbes on plant roots and stones remove nutrients and contaminants, and free-swimming micro-organisms eat algae.

natural-swimming-pools-regeneration

Regeneration area with water and bog plants and plants with floating leaves, with a partition wall to separate it from the swimming area, but to allow the water to flow between the two areas.

History

The modern natural swimming pool ‘movement’ is considered to have started with Gottfried Kern in Austria in 1954. He partitioned free-standing water from a ‘regeneration zone’, where plants and animals could live undisturbed, but could also venture out into the swimming area. The 60s saw a wave of chemically-treated swimming pools spread across Europe from the US, but in tandem with the wildlife gardening movement in the 70s, more and more people in central Europe were looking at installing natural pools for bathing. Installation companies were formed in the late 80s, and a new sector developed in the 90s.

digging

Two very different methods of digging the hole for your pool.

Legal and technical barriers had to be overcome to enable the step from private to public natural pools. The first public natural swimming pool, created by Biotop, was opened in Klosterneuburg, Austria, in 1991. Natural swimming pools arrived in the UK in 2001 – the first was at a private residence in Gloucestershire by Michael Littlewood. In 2015, the UK’s first public natural swimming pool was opened at King’s Cross in London.

Types

Pools can have different levels of wildlife-friendliness. Some pools, although unchlorinated, have vertical walls, with no escape route for amphibians, and intensively-filtered water that reduces plant growth.

birds-eye-view

Bird’s-eye view of a natural pool.

Wolfram Kircher lists four main types of natural swimming pool, by increasing complexity (and therefore cost):

  1. Swimming pond: just a body of water with shallower edges (the regeneration zone) for water plants (including reeds and wetland plants, submerged plants and plants with floating leaves) that do all the filtering, but without a pump. This is the most cost-effective model, but you’ll need quite a large area (at least 120m²) for it to work effectively.

  2. Swimming pond with surface flow: a skimmer or overflow gutter allows water to flow from the surface to a pump, which oxygenates and delivers the water back through the regeneration area to the swimming area. This provides a constant nutrient flow for the plants. At least 50% of the surface area should be planted, and the overall area should be at least 100m².

  3. Pool with ‘technical wetlands’ that operate like a reed bed: a pump circulates water to a vertical- or horizontal-flow gravel bed that traps nutrients and makes them available to plant root systems (usually reeds), which keep the gravel beds permeable. Water passes through the beds back to the swimming area. You could even pump water to the top of a series of gravel-filled terraces, creating a rock garden to filter the water.

  4. Pool with biofilm-accumulating substrate filter (BSF): this is a gravel bed whose coarser stones are kept aerated, and through which circulating water can percolate quickly, so that they become covered in a ‘biofilm’ of micro-organisms that remove nutrients and pathogens from the water filtering through it. No reeds or other plants are required (and in fact would find it difficult, because nutrients are taken by the biofilm). Water filtered through the biofilm filter bed is pumped back to the swimming area.

Types 3 and 4 can be much smaller than types 1 and 2. You can also have pools that combine features from two or more of these models.

DIY swimming pond that was dug by hand and is kept clean by a combination of water plants and an ingenious home-made biofiltration system; cost – around £2000 in total.

What are the benefits of natural swimming pools?

For people:

To guarantee pathogen-free water, traditional swimming pools are usually treated with chlorine, chlorine dioxide, mineral salts, organic biocides or ozone. Chlorine increases the risk of asthma and allergies, especially for kids. It is also a potential carcinogen. Regular exposure to chlorine may cause reproductive disorders and birth defects as well as skin ageing. A natural swimming pool does not require added chemicals. Water purification is achieved via physical and biological processes.

humans-frogs

Nothing better than plunging into a freshwater pool on a hot summer day – for humans and non-humans.

Of course, very few natural pools will be large enough for lane swimming, but they’re fine for swimming and splashing about in the fresh air, and they’re great as plunge-pools after a sauna.

For nature:

A natural swimming pool will provide habitat for wildlife. Not everyone will be happy to share their swimming space with frogs or water beetles, but they’ll try to get out of your way, and if you’re a nature lover, you’ll take to it easily. The environment will benefit from the fact that chlorine isn’t used, and it will save water too, as, unlike conventional swimming pools, total water changes are not required.

natural-swimming-pools-liner

Unrolling the pool EPDM liner.

What can I do?

Type 1 and 2 pools can easily be built DIY – you don’t need much more knowledge than for a normal garden pond. You could also have a go at type 3, but you’ll probably need a professional for type 4. You’ll need to consult a good book for more detailed information on construction, stocking with plants and maintenance.

conversion

This is a conversion from a conventional pool – chlorine out, plants in.

Construction

Here are the (basic) construction stages:

Planning: choose the type, location, size, depth and partitioning of the pool. Think about filtering equipment and adjacent decks, paths, planting areas etc.

Excavation: you can hire a mechanical digger, or you could dig it by hand, using picks and shovels. It’s a big job – definitely a case of green gym. The swimming area needs to be at least 2m deep.

Underlay and sealing: EPDM (synthetic rubber) is considered more environmentally-friendly than PVC liner, and definitely more so than concrete – plus it’s UV resistant. It will need an underlay – old carpets can come in handy.

Build superstructures: add things above the liner, like stone steps, foundations for wooden decking or bridges, boulders or flag stones on the bottom of the pool, and around the edge of the liner; you can also start filling the main swimming area with water to tension the liner.

Build regeneration zones: 50-150cm deep, and separated from the main swimming area with an underwater barrier wall that allows water to flow over the top of it into the swimming area; introduce plants (beginning in the deepest zone), then fill up with water in stages.

Construct the capillary barrier: the edge of the liner must be above the level of the surrounding soil, and not in contact with it. This is to stop the drier soil sucking water from the pool, and also to prevent influx of surface water after heavy rainfall, which might wash in nutrients and pollutants.

You won’t need planning permission for a natural swimming pool unless you’re in an area of outstanding natural beauty (AONB) or a conservation area, although even then you might not. Best to contact your local authority if you’re in one of those areas. Note that existing conventional pools can be converted to natural pools – plants replace chlorine when it comes to keeping the water pathogen-free.

DIY pool with a solar-powered filtration system, costing a fraction of the price of a conventional swimming pool, and providing a home for newts and other wildlife. This booklet explains how it was built.

Welcoming wildlife

Stocking with animals is not recommended. Fish will raise the phosphorus levels and increase the likelihood of algae. Wildlife, however, will come on its own. A few days after filling the pool, you might get an algal bloom that could make the water murky; but soon after that, the first microscopic animals such as water fleas and daphnia will appear to eat the algae and the pool will clear again. Their predators – water-insects and arthropods – will inevitably follow: pond skaters walk over the water surface thanks to their hydrophobic feet; great diving beetles fly in, as do whirligig beetles with their fantastic dances on the surface of the water. Later, dragonflies will lay their eggs, and their larvae will go hunting with their extending mandibles. Later still, frogs, toads and newts will find their way overland to lay their eggs. All very exciting if you enjoy observing nature.

daphnia

After you’ve built your pond, you may start to get a build-up of algae, but soon daphnia (above) will arrive to eat it; and after that, other pond life will turn up to eat the daphnia, and an ecosystem will develop.

Plants are important for filtering in pools of type 1 and 2. This includes a densely planted area with submerged plants and floating-leaf plants. Plants withdraw nutrients, raise oxygen levels and provide a habitat for beneficial micro-organisms that kill pathogens. Plants for technical wetland areas (type 3 pools) are mostly helophytes such as common reed (Phragmites australis), cattails (Typha spp.), irises (e.g. Iris pseudacorus) or sedges (e.g. Carex elata). In this case the roots and rhizomes keep the filter aggregate permeable for water percolation and promote the development of beneficial micro-organisms. In pools with fast percolation (biofilm accumulating substrate filters, type 4) plants are only for decoration. Consult specialist books for the range of native plants that are available.

biofilm-gravel

Constructing a gravel bed for a ‘biofilm’ filter (type 4 pool, see above).

Maintenance

Maintenance is necessary for all types of pool. The accumulated mud has to be removed with a suction pond cleaner at least once per year. They’re quite cheap, and can transfer the pond mud to the garden, where it will be a good fertiliser.

Plants must be pruned back each autumn to remove nutrients. Some weeding might have to be done too, as well as possibly removing accumulations of filamentous algae.

Skimmers and/or pumps, if used, will include mechanical filters that capture leaves, dirt, pollen etc. These need to be cleaned out – keep checking them and you’ll work out how regularly they need to be cleaned (which will change depending on the season).

Yellow flag irises can add beauty to natural swimming pools

Yellow flag irises love wet conditions and will add beauty to your pool, as well as helping to keep the water clean.

Occasionally, the biofilm on the gravel in type 4 pools will reach its final phase of ageing. Particles die off and are flushed into the open pool area, making the water murky. To prevent this, backwashing is required. The pump is switched off for a week to kill all biofilm microbes. Then, the direction of water flow in the distribution pipes is reversed – instead of pumping water into the filter, it is drawn off and allowed to trickle away or irrigate the garden. This discharged water will initially be brownish and murky or smell mouldy. As soon as it becomes clear and loses any unpleasant smell, the pump can be switched back to its original setting, and a new biofilm can begin to establish itself. Backwashing is recommended at least twice a year, in spring and autumn. An additional backwashing can be beneficial in mid-summer.

Thanks to Wolfram Kircher, author of How to Build a Natural Swimming Pool for information.

 


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Wolfram Kircher is an international expert on natural swimming pools and author of How to Build a Natural Swimming Pool. He teaches landscape architecture and planting design at Anhalt University, Bernburg, Germany. His research into low-maintenance naturalistic plantings on extreme substrates, which made an important contribution to “new German style” planting, puts habitat planting in reach of amateur gardeners. He lectures widely.


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